The microcircuitry of the adult cat retina has been studied and described in terms of its physiology, cell morphology and synaptic relationships. However, the sequence of development and maturation of the neural processes and their connections is unknown. The ontogeny of identified neurons of known date of final mitosis ("birthday") has been studied in the kitten retina in our laboratories, using a novel combination of techniques. In this approach, replicating cells are labelled with an intravitreal injection of tritiated thymidine and cell morphology is delineated by subsequent in vitro staining of the cell with the fluorescent dye Lucifer Yellow. Preliminary experiments have focussed on the period of postnatal genesis and maturation of retinal interneurons. Initial studies in a limited number of animals, using the combination of tritiated thymidine labelling and Lucifer Yellow staining, have demonstrated that the majority of neurons born on postnatal day 2 (P2) are bipolar cells and B type horizontal cells; the maturation of their morphology has been followed at days P9, P23, and P37. Additional experiments are planned which would examine neurogenesis at other times in the first three postnatal weeks. The experiments proposed will expand these preliminary studies to include cells, such as beta ganglion cells, with prenatal birthdays. The project will be extended to the ultrastructural level in a study of the synaptic relationships of identified neurons in the postnatal reorganization of the inner and outer plexiform layers. These experiments will provide previously unavailable information on the normal development and maturation of individual cell types within the neural circuitry of the mammalian retina. A specific application would be better understanding of the retinal defect in amblyopia, the reduction in visual acuity caused when retinal images are blurred during the critical period of retinal development.